TEGUMENT ARY ORGANS. 



475 



as it might be termed, external and internal 

 to which, is a zone of indifferent tissue, while, 

 .still more remote again, is a zone of meta- 

 morphosed tissue. The absolute structure of the 

 two layers thus produced is very similar*, so 

 much so, that, as is well known, either may 

 perform for a time the function of the other. 

 The distinction between the integument and 

 the mucous membrane in a morphological 

 point of view, however, is as strongly marked 

 as in the most complex animal. The integu- 

 ment, in fact, grows from within outwards it 

 is endogenous, its youngest portions being 

 internal : the mucous membrane, on the 

 other hand, grows from without inwards 

 its youngest portion is external, and it is, 

 therefore, exogenous. 



We have here, I believe, the fundamental, 

 and the only essential distinction, between 

 true integumentary or "epidermic" structures 

 and all others. An integumentary or epidermic 

 organ forms or has formed a part of the external 

 surface, and grows endogenously ; its youngest 

 portion and plane of no differentiation being 

 directed inwards. 



If, for instance, we compare the young skin 

 of a mammal with the body of the Hydra, we 

 shall find precisely the same planes and zones. 



Fig. 303. B, represents a perpendicular sec- 

 tion of the integuments of a foetal Iamb 3^ 

 inches long. (A) marks the position of the line 

 of no differentiation separating the epidermis 

 from the derma ; on the outer side of that 

 line lie the close-set endoplasts of the deepest 

 layer (rete) of the epidermis, which are dis- 

 posed somewhat perpendicularly to the sur- 

 face. On the inner side are the less approxi- 

 mated endoplasts of the outer youngest layer 

 of the derma, more or less parallel to the sur- 

 face. From a to b, lies the epidermic area of 

 metamorphosis, the indifferent tissue becoming 

 gradually converted into flattened horny cells. 

 From a to c, on the other hand, is the dennic 

 area of metamorphosis, the indifferent tissue 

 gradually changing into connective tissue. 



It will be observed here, that as the whole 

 serous layer of the germ corresponds in struc- 

 ture with the epidermis only, of the fully 

 formed animal, so the whole integument of 

 the Hydra corresponds with what is usually 

 considered as only a portion of the integu- 

 ment the epidermis of the mammal. The 

 derma, or true skin of the latter, would not 

 come at all under our present definition of 

 integument, since it has all the morphological 

 characters of the mucous layer of the Hydra, 

 or of the germ ; i. e. its youngest layer is ex- 

 ternal, its growth is exogenous, and the me- 

 tamorphosis of its tissue takes place from 

 within outwards. 



In fact, in all animals higher than the 

 Hydroid Polypes (possessing therefore a vis- 

 cera! cavity) we find a complication of struc- 

 ture, corresponding with that which is pro- 

 duced in the germ, when the "membrana in- 

 termedia" divides into its parietal and intes- 

 tinal laminae. Compared with the Hydroid 



* Though not, as it is commonly said, identical. 



Polypes, the higher forms are double animals, 

 and a section of their bodies is, morpholo<n- 

 cally speaking, like a section of two Hydra?, 

 one contained within the other. Both the 

 intestinal parietes, and those of the body, pre- 

 sent the same distinction into a central plane 

 of no differentiation, from which growth and 

 metamorphosis proceed inward and outward 

 on the two respective surfaces, as that ob- 

 served in the parietes of the Hydra. 



The formation of this so-called membrana 

 intermedia, in fact, appears to result from a 

 repetition of the process which gave rise to 

 the two primary layers of the germ. The 

 previously central plane of no differentiation is 

 replaced by two others, from which growth 

 and metamorphosis proceed in the same way. 

 The result is, of course, the division of the 

 germ into three layers a central and two 

 superficial (inner and outer) planes of meta- 

 morphosed tissue and two planes, whence 

 growth and metamorphosis proceed. 



It results from all this, that, among the higher 

 animals, the true homologue of the integu- 

 ment of the Hydra is the epidermic layer 

 alone. But it \vouldbeexceedinglyinconvenient 

 to change the accepted meaning of " Integu- 

 ment " on this ground ; and, therefore, I shall, 

 throughout the present article, consider as 

 integument the outermost plane of indif- 

 ferent tissue in the animal body, with its external 

 and internal arece of metamorphosis collectively; 

 these being simply the expressions of two pro- 

 cesses of growth in opposite directions, and their 

 line of contact. 



It must not be supposed that this phrase- 

 ology involves any hypothetical views: the fact 

 that any integumentary organ consists of 

 these three portions will be found to be either 

 distinctly stated or implied by all writers, 

 and is indeed obvious enough on inspection. 

 But though the facts be old enough, this ex- 

 pression of them is unfortunately so new, that 

 I know of no existing terminology by which 

 it can be properly enunciated. The term 

 " Epidermis," tor instance, at present, though 

 it denotes the important character of the 

 direction of growth to which I refer, implies 

 even more strongly the simple cellular struc- 

 ture of an organ; so that to speak of "Epi- 

 dermic" bony or fibrous tissue would sound 

 almost contradictory. Again, all these distinc- 

 tions, which have been shown to exist between 

 the two elements of the integument, equally- 

 bold good with regard to the mucous mem- 

 branes. Now we have a term " Epithelium " 

 for the epidermic element of the latter ; but 

 there is, as far as 1 know, none for the ele- 

 ment which corresponds with the derma. Nor 

 have we any word for the boundary line be- 

 tween the endogenous and exogenous area; 

 of growth the term "basement membrane" 

 expressing only an accidental character of the 

 tissue immediately on one or the other sides 

 of that line. 



Although with great reluctance, then, I feel 

 compelled to propose two or three new terms, 

 which may have general application, not only 

 to the integumentary organs, but to all other 



